The use of hyper-abrupt heterojunctions, on the order of as few as one atomic diameter, has enabled advanced heterostructure devices to be realized. One of the more advantageous structures is the high electron mobility transistor(HEMT) structure. A particular class of these devices is the Pseudomorphic HEMT, or PHEMT device. These are higher performance devices than their counterpart the MESFET. The PHEMT structure has higher gain than the MESFET, and this results in power devices with higher efficiency and thereby higher power capabilities which has particular ramifications in the cellular phone business which is ever seeking lower DC voltage levels for operation. Another desirable attribute of PHEMTs is a relatively fast on/off cycle. In the "on" state, due to the high conductivity of the channel, there is low signal loss through the device. In the "off" state, the combination of lower pinch-off voltage and higher breakdown voltage translates to more truly "off" behavior with reduced signal leakage between terminals. The transition from on to off of the PHEMT is relatively quick, particularly when compared to the MESFET counterpart. The basic Pseudomorphic HEMT structure uses a high purity/high mobility InGaAs material that is not intentionally doped for carrier transport. Doping introduces scattering centers, which reduce carrier mobility and velocity as is well known to one of ordinary skill in the art. At equilibrium, the heterojunction between the wide band gap material and the narrow band gap (undoped/high mobility) material creates a quantum well in the narrow bandgap semiconductor material. Electrons from the high band gap material tunnel through the energy barrier in the higher bandgap material into the quantum well. This charge transfer forms a sheet of electrons, known commonly as a two dimensional electron gas (2DEG). These 2DEG electrons in the undoped narrow band gap material possesses a very high mobility and velocity.
Conventional HEMTs rely on the hyperabrupt heterojunction between AlGaAs and GaAs. As stated, such a structure lends itself quite well to the fabrication of high electron mobility transistors which make use of the 2DEG electrons in the GaAs channel. Another material which has come into prominence in 2DEG devices is InGaAs. The heterojunction between InGaAs and AlGaAs also forms the two dimensional electron gas by virtue of the quantum well formed between the narrow band gap InGaAs and the wide band gap AlGaAs. InGaAs is also a material which when undoped has a very high electron mobility. The electron mobility and peak electron velocity of InGaAs is in fact much higher than GaAs. Due to differences in lattice constants of the two materials, epitaxial growth of thin InGaAs layers on GaAs substrates results in considerable strain in the InGaAs layer. This strain further deepens the quantum well formed increasing the number of electrons in the 2DEG of the Pseudomorphic HEMT, as is well known to one of ordinary skill in the art. Lattice mismatch between GaAs and InGaAs increases with Indium concentration and hence deepens the quantum well. Lattice mismatch is desirable to a degree, however if indium is introduced in too high a concentration, InGaAs lattice relaxation can occur. The lattice relaxation is the state when accumulated strain is too great and is relieved through the formation of lattice defects which behave as scattering centers. These scattering centers have a deleterious effect on carrier mobility. It has been found that molar fractions of indium to gallium of 53 to 47 are desirable. That is, In.sub.0.53 Ga.sub.0.47 As results in the material with a very high peak electron velocity, a relatively deep quantum well with more carriers disposed therein. However, due to severe lattice mismatch, an epitaxial structure containing In.sub.0.53 Ga.sub.0.47 As can only be grown on an indium phosphide substrate and device fabrication of InP based materials is relatively immature compared to GaAs. The mole fraction of indium is preferably 15-25% when InGaAs is grown on a GaAs substrate; beyond this lattice relaxation can occur thereby reducing carrier mobility and velocity.
The desirable performance advantages of the PHEMT structure by virtue of the high mobility of the carriers of the two dimensional electron gas which is disposed in the InGaAs epitaxial layer results in a higher electron saturation velocity compared to conventional MESFET structures. Because the high mobility two dimensional electron gas is disposed deep in the epitaxial layer structure, relatively complicated techniques have to be employed in order to be able to modulate the 2DEG layer with a gate. A conventional PHEMT structure is as shown in FIG. 1. The structure shown in FIG. 1 has a gate 101 a source 102 and a drain 103. The n.sup.+ GaAs layer 104 is highly doped to effect the Ohmic contact at the drain and source. The AlGaAs layer 105 and the InGaAs layer 106 form the required heterojunction to form the PHEMT structure. The substrate is shown at 108, and the 2DEG conduction layer is shown at 107. The gate recess etch is a very important step in the fabrication of all GaAs base field effect transistors. This step determines all the critical DC parameters of the device, whether the device is a PHEMT or a MESFET.
Conventional techniques for fabricating the recess require an iterative etching process. As can be readily appreciated, it is necessary to reduce the distance between the gate and the 2DEG layer to an optimum point in order to effectively modulate the 2DEG layer in operation by way of the depletion layer formed under the gate. As stated above, a double recess structure is often utilized in the PHEMT. An iterative fabrication process requires etching down through the GaAs and AlGaAs layers while sampling the current periodically. As the etching process proceeds, carriers are removed as selective regions of each layer are removed. As can be seen in FIG. 2, which is a graph showing the current versus the depth of the etch, a large decrease in saturation current per unit of depth occurs in the first region 201 which is n.sup.+ GaAs. The region 202 shows a reduction in the slope of the saturation current relative to the depth, as the number of carriers in the lower doped AlGaAs layer is etched. Finally, the optimal point of etch depth is shown at 203. Beyond this point, a large number of carriers would be removed from the 2DEG layer and is shown FIG. 2 at 204. Accordingly, it is necessary to etch down far enough to be able to effectively modulate the 2DEG layer but not too far as the benefits of the layer are reduced as the etched surface becomes too close. Accordingly, the optimal point is at the knee of the curve shown at 203.
As stated, the double etch is required in order to increase the breakdown voltage of the PHEMT. Standard photolithographic techniques are used to effect both the first etch recess and the second etch recess. Further details of the iterative etching process done conventionally can be found in Effects of Material Variations on the Gate Recess Behavior of Pseudomorphic HEMTs, by Danzilio et, 1994 US Conference on GaAs Manufacturing Technology Digest of Papers, p. 53, the disclosure of which is specifically incorporated herein by reference.
Another important consideration is the effect of material variations. As stated in the reference to Danzilio, et al., variations of the vendor molecular beam epitaxy(MBE) can result in inconsistent epitaxial growth which appreciatively alters the characteristics of the PHEMT. Accordingly, while the etch depth for one wafer might be a certain value, this is not necessarily the appropriate depth in another wafer. Accordingly, rather than being at the proper point 203 of the curve shown in FIG. 2, it is possible to stop etching prior to reaching this point (for example in the region of 202) or to etch too far and end up in the region 204. Material variations play an extremely important role in the double recess etch of the PHEMT structure. If the depth of the second recess is too shallow relative to the first recess etch, gate control can be substantially reduced. To this end, if the second etch depth is relatively shallow compared to the first, the depletion region underneath the gate when the gate is swung as far in forward bias as possible is reduced enough that conduction in the 2DEG layer is uninhibited by this depletion region. However, the regions on either side of the gate about the first etch level have depletion regions thereunder due to surface charges. These depletion regions can actually impede carrier flow in the 2DEG layer, and result in a limit in the open channel current.
One technique to overcome the disadvantages of the interative process has been through the use of etch-stop layers. Single or double etch-stops can be used to properly control the depth of the etches so that both the depth of the second etches as well as the relative depth of first and second etches of the double recess structure are optimized to overcome the enumerated problems of the double recess structure and the iterative etching process to achieve PHEMT fabrication. Prior techniques include the use of AlAs, which enables the selective etching process to be carried out and the appropriate depth to be reached, but adversely impacts the access resistances. As is discussed in copending applications U.S. patent applications Ser. Nos. (09,121,144 and 09/121,160) entitled "In.sub.x Ga.sub.1-x P Stop Etch Layer For Selective Recess Of GaAs Based Epitaxial Field Effect Transistors" And "A Process For Selective Recess Etching Of Epitaxial Field Effect Transistors With A Novel Etch-Stop Layer", respectivley, to Hanson, the use of AlAs as the etch-stop layer is undesirable by virtue of the increase in access resistances and the reduction therefore in efficiency of the device. The disclosure of the above captioned patent applications to Hanson are specifically incorporated herein by reference.
Accordingly, what is needed is a PHEMT structure which has the benefits of the double recess with precision in both the overall etched depth as well as in the relative etch depth of the first and second recesses without the deleterious effects of prior art etch stops.